The present invention relates to interbody spinal implants for placement between adjacent vertebral bodies of a human spine, and more specifically to a specialized surface for such interbody implants, for engaging the adjacent vertebral bodies. Vital to the functioning of all interbody spinal implants is their ability to remain properly located within the spine after installation. In U.S. Pat. Nos. 5,593,409 and 5,609,635, Michelson described the use of surface roughenings, such as knurling or ratcheting on the opposed upper and lower vertebral body engaging surfaces of interbody spinal fusion implants. Knurling has been particularly beneficial for increasing the grip of the implant surface to the adjacent vertebral bodies in a rather uniform manner without a directional bias. Spinal implants have a propensity to move in a particular direction, which is opposite to their path of insertion, because this is the path of least resistance. Such propensity to move is further increased when the opposed upper and lower vertebral body engaging surfaces are in angular relationship to each other, such that they are spaced further apart at the implant""s trailing end than at the implant""s leading end. In such circumstances where it is desirable then to gain stability in resistance to a particular direction of movement of the interbody spinal implant, the use of a plurality of forward facing ratchetings on the implant""s vertebral body engaging surfaces has been preferable to the previously described knurling for that purpose.
The term xe2x80x9cratchetingxe2x80x9d as used herein is defined as a plurality of angular teeth or ridges or protrusions projecting from the surface of an implant to resist motion of the implant at least in one direction. The phrase xe2x80x9cforward facing ratchetingsxe2x80x9d as used herein is defined as a ratcheting having at least one forward facing facet having a length greater than a rearward facing facet and an angle from the implant surface from which the forward facing facet arises that is less steep than the angle of the rearward facet. On an implant surface, forward facing ratchetings facilitate the insertion of the implant in one direction and after insertion, resisting movement of the implant in a direction opposite to the direction of insertion. An example of forward facing ratchetings of the prior art is shown in partial fragmentary view in FIGS. 24A and 24B, generally referred to by the reference numeral 50.
Knurled surfaces of the related art provide some stability in all directions, but lack the ability to resist a particular direction of motion preferentially. The above-described ratcheted surface best resists motion in a particular direction. There exists a need for an improved interbody spinal implant surface configuration, wherein the opposed upper and lower vertebral body engaging surfaces of the implant are configured to be resistant to implant movement in all directions, and preferentially or in particularly in one direction.
The present invention relates to interbody spinal implants having a specialized surface configuration on the opposed exterior surfaces adapted for engaging the vertebral bodies adjacent a disc space into which the interbody implant is to be implanted. Such an implant surface configuration has utility with a wide variety of shapes of interbody spinal implants where enhanced skeletal fixation is desired. Such an implant surface configuration can provide for enhanced stability, increased surface area, and a surface for the delivery of fusion promoting substances other than bone. In a preferred embodiment, the implant surface can provide for resisting motion in all directions, and particularly in at least one direction, such as counter to the direction of insertion of the implant.
While various embodiments of the present invention are presented by way of example only and not limitation, common to each of them is that the surface configuration incorporates a plurality of spatially integrated surface projections having at least one forward facing facet directed at least in part toward the leading end of the implant and at least one rearward portion directed at least in part toward the opposite trailing end of the implant. By way of example and not limitation, the rearward portion may be a facet, a line, or an edge of the rearward aspect of the surface projection formed where two facets come together. Each of the forward and rearward facets have a length and a slope. The length of the forward facet is longer than the length of the rearward facet. The slope of the rearward facet is steeper than the slope of the forward facet. In various embodiments, the surface projections also have opposed side facets directed generally toward the sides of the implant. The side facets are located between the forward facet and rearward facet and may converge toward each other in a direction away from the base of the surface projections. The surface comprises multifaceted ratcheted projections that are organized in geometrically disposed fields or arrays which are at a minimum located on at least a portion of the opposed vertebral body engaging surfaces of the implant. From the teachings disclosed herein, it is appreciated that the surface projections can be geometrically arranged in a pattern wherein at least a portion of the projection is aligned along a longitudinal, horizontal, diagonal, or curved line. The upper and lower surfaces of the implant can be at least in part arcuate or planar and can converge along a portion or all of the length of the implant.
In various preferred embodiments of the present invention, the rearward facets of the surface projections can be perpendicular or at angles greater or less than 90 degrees to at least one of the upper or lower surfaces of the implant from which the projections arise. The opposed side facets of the surface projections can have at least a first portion in a plane at an angle to the longitudinal axis of the implant. The opposed side facets can intersect each other, and can converge to form a peak at the top of each of the surface projections. The peaks can be aligned along lines that are perpendicular, parallel, or diagonal to the longitudinal axis of the implant. The surface projections can be cleaved such as by longitudinal and/or horizontal cuts to increase the number of exposed sides of the projections and thus increase the available surface area to contact and engage the bone of the adjacent vertebral bodies and increase the number of recessed areas to contain fusion promoting substances. Alternatively, the peaks of each surface projection can be cleaved, truncated, or flattened at least in part.
The surface projections can include a left forward side facet and a right forward side facet directed toward the leading end and sides, respectively, of the implant. Similarly, the surface projections can include a left rearward side facet and a right rearward side facet directed toward the trailing end and sides, respectively, of the implant. The side facets of adjacent surface projections can be spaced apart to define a groove therebetween. A plurality of adjacent surface projections can be spaced apart to form a plurality of grooves that can be parallel or at an angle to the longitudinal axis of the implant, wherein the angle can be less than 90 degrees. The grooves can have a horizontal cross section that is a V-shape, U-shape, or a box-like shape, for example.
Sequential projections can be positioned on an implant wherein each surface projection has forward facing facets facing the same direction, such that consecutive projections are oriented forward facing facet to rearward facing facet. The lower most portion of the slope of the forward facing facet of a first surface projection in a sequence can be coincident with the rearward facet of the next surface projection in the sequence. Alternatively, the forward facet of a first surface projection and the rearward facet of the next surface projection in a sequence can be spaced apart and the space can be at least in part flat, curved, or any other surface contour suitable for the intended use. The surface projections can be oriented relative to one another to form fields or arrays that further can be geometrically disposed relative to one another, preferably in a pattern wherein at least a portion of the projection is aligned along a longitudinal, horizontal, diagonal, or curved line.
The surface configuration of the present invention can be formed by casting, machining, or any other techniques known to one of ordinary skill in the art. The present surface configuration may readily be machined by milling the implant surface from side to side, across the upper and lower vertebral body engaging surfaces, to form ratchetings generally disposed perpendicular to the long axis of the implant and generally formed facing to the insertion end of the implant. The ratchetings may be cross machined with an angled cutting face to form grooves passing through the ratchetings. For example, a milling machine having a cutting tool, with a V-shaped profile, can be run through the plurality of ratchetings parallel to the longitudinal axis of the implant to form the above-described surface. In a preferred embodiment, the V-shaped cutting tool of the milling machine has opposed cutting faces with an angle of approximately 90 degrees to each other, which faces are each at a 45-degree angle to the plane of the surfaces being machined. Without departing from the scope of the present invention, the angle of the cutting faces can be more or less than 90 degrees, and the angle of the cutting face to the surface to be cut can be more or less than 45 degrees. It is appreciated that rather than the cutting element being run parallel to the longitudinal axis of the implant, the cutting element could be run at some other angle. By way of example only and not limitation, this angle could be at 45 degrees to the longitudinal axis of the implant and to the projections. Each surface projection could then be formed by a cutter crossing in two passes to form two grooves at a 90 degree angle to each other.
The surface of the present invention for engaging each of the adjacent vertebral bodies may be incorporated into various types of spinal implants. Such spinal implants may be for the purpose of achieving interbody spinal fusion, or for stabilizing a device to space apart and allow motion between the adjacent vertebral bodies. Such spinal implants may comprise any artificial or naturally occurring material appropriate for the intended purpose. Such materials would include, but are not limited to, implant quality metals, including, but not limited to, titanium and its alloys, surgical grade plastics and plastic composites which may or may not be bioresorbable, ceramics, and cortical bone. Some examples of interbody spinal implants that may benefit from the present teaching, include but are not limited to the following patents and applications by Michelson which are incorporated by reference herein: U.S. Pat. Nos. 5,015,247; 5,522,899; 5,593,409; 5,609,635; 5,860,973; and application Ser. No. 08/480,904.